The present invention relates to a heat-conducting cooling module for cooling a semiconductor substrate in an integrated circuit package assembly in which a semiconductor substrate is mounted on a base board by small solder pellets, and which contains a single substrate or laminated substrates.
It is essential for large electronic computers to operate at high speeds and, for this purpose, a semiconductor device or a large-scale integrated circuit (hereinafter referred to as LSI) chip has been recently developed in which a number of semiconductor elements are packed onto a semiconductor substrate, in order to reduce the length of electric wiring among the elements. A circuit board on which the LSI chip is mounted has also been developed, wherein base boards are laminated and are provided with electric wiring that is very densely formed to electrically connect the LSI chip to external circuits. Moreover, a method of mounting a number of LSI chips on the circuit board has also been developed. To maintain operation parameters of the LSI chip within predetermined ranges and to prevent the LSI chip from being destroyed by overheating, the heat generated by the operation must be efficiently externally radiated.
When the density of mounting the LSI chips is low and little heat is generated, it is accepted practice to employ a cooling method of the type in which a heat radiating member is cooled by the forced convection of air, with the heat radiating member being connected, via a heat relay medium, to the LSI chip that is a source of heat. However, as the density of mounting the chips increases and heat is generated in increased amounts, it becomes necessary to sufficiently cool heat radiating members having limited surface areas and for this purpose, the rate air flow must be increased. This, however, creates noise. Therefore, it is no longer possible to maintain LSI chips within a suitable operating temperature range unless some auxiliary cooling means is provided to replace the forced air-cooling system.
An auxiliary cooling means represented by a cooling system based upon air and liquid cooling is proposed in, for example, U.S. Pat. No. 3,741,292, wherein a module is provided in which heat-generating components are immersed in a portion surrounded by a low-melting dielectric liquid that is contained in a capsule. The liquid is a fluorocarbon liquid having a low melting point, i.e., which boils at a relatively low temperature. That is, the liquid, receiving the heat from the heat-generating component turns into a vapor and migrating into a vapor portion located above the liquid level, is cooled as it passes through internal fins that are formed continuously from the container to the interior thereof and that work as a condenser, and is liquefied, thus forming a repetitive cycle. In this case, the external fins stretching from the container to the outside are air cooled to radiate the heat transmitted from the internal fins. Thus, the heat generated by the heat-generating components is radiated.
In a module containing a liquid capsule of this type, however, the fundamental process, i.e., boiling condensation, must be reliably executed. For this purpose, the cooling agent or liquid must be extremely pure and free of contaminants. The above-mentioned cooling concept cannot be easily adapted to cooling heat-generating components such as LSI chips. This is because, the substances constituting the LSI may be corroded by the liquid and impurities or contaminants in the liquid, are very likely to cause malfunctions.
In U.S. Pat. No. 3,993,123, a cooling module is proposed wherein one or more heat-generating devices such as semiconductor chips to be cooled, are contained in a capsule together with a gas. The heat-generating devices are mounted on an alumina plate and are sealed by the plate and by a cap. The space thus sealed is filled with an inert gas, and a heat-conducting member is arranged therein. Apertures are formed in the wall surface of the cap opposed to the plate with the apertures facing the heat-generating devices and stretching on axes concentric with the devices. An elastic member is disposed at the inner end of the aperture. The heat-conducting member is disposed in each of the apertures, and a narrow peripheral gap is formed between the wall surface of each aperture and the heat-conducting member. The elastic member imparts to the heat-conducting member a force such that the heat-conducting member is pressed onto the heat-generating device. The sealed space is filled with inert gas; i.e., the peripheral gap and the interface between the heat-generating device and the heat-conducting member are filled with inert gas. The heat generated by the heat-generating components is conducted to the gap via the inert gas and heat-conducting member, and is emitted to a heat sink that is coupled to the cap.
In the above-described capsule-sealed cooling module, the cap and the heat-conducting member are made of copper or aluminum, and the gas is composed of helium, hydrogen or carbon dioxide which have excellent thermal conductivity. By constituting heat-conducting paths from the heat-generating devices to the heat sink using these materials, the heat can be radiated highly efficiently. However, the cap and the heat-conducting member accounts for more than 50% of the total weight of the capsule-sealed cooling module. For instance, when a typical cooling module mounting ninety LSI chips, which heat-generating devices, is to be efficiently cooled, the weight increases to as much as 1.5 kg if copper is used to form the heat-conducting members and caps. The cooling module is generally connected by soldering to a printed board which carries auxiliary circuits via many connecting pins that protrude perpendicularly beyond the surface opposite to the surface of the board mounting the heat-generating devices, and that are secured by a gold-tin brazing or the like. Furthermore, the cooling module is so mounted that the surface of the printed board is in parallel with the pulling direction. Therefore, the total weight of the cooling module is supported by the pins and members connected thereto, such as a gold-tin-type brazing material or a solder. In operation, the cooling module receives a mechanical vibration that is transmitted from a housing that mounts the module via the printed board. However due to an excess load, the connection pins and connection members supporting the cooling module mechanically deteriorate at an accelerating rate, thereby causing malfunctions.
According to the above-mentioned typical cooling module, furthermore, the heat-conducting member has a weight of about 3.3 grams to maintain efficient cooling ability. Vibration in a direction perpendicular to the main surface of the heat-generating device is absorbed by the elastic member. However, vibration in a direction parallel to the main surface is absorbed little by the elastic member. That is, a gap exists between the wall surface of the aperture and the heat-conducting member, and the heat-conducting member composed of soft copper wears out at the point of contact with the heat-generating device accompanying the vibration. Accordingly, the electrically conductive powered material in the sealed module adheres to the electrically conductive paths that are electrically insulated from one another, thereby destroying the insulation, so that the device malfunctions. Similar wear also takes place between the wall surface in the aperture of the cap and the heat-conducting member. Furthermore, the problem of wear also takes place when aluminum is used for the cap and the heat-conducting member.
Further, the vibration of the heat-conducting member does more than trigger the slipping motion on the contacting surfaces between the heat-generating device and the heat-conducting member. For instance, when a resultant vibration, consisting of a component perpendicular to the main surface of the heat-generating device and a component in parallel therewith, is imparted, the perpendicular component of the resultant vibration may not be absorbed by the elastic member if the heat-conducting member is too heavy. Therefore, the heat-conductive member comes into point or line contact with the main surface, thereby decreasing thermal conduction. Moreover, pressing force is concentrated onto, or shock is imparted onto, a local portion of the heat-generating device. In this case, connecting members such as small solder pellets connected to the heat-generating device or connecting said heat-generating device to said board, deteriorate, i.e., they are cracked, split, or crushed at an accelerated rate. This problem is very noticable when the heat-conducting member is very heavy or when the tolerance of the gap increases between the cap and the heat-conducting member.
Further, heat-generating devices are mounted as densely as possible on the board in the sealed capsule. Each heat-generating device has a number of semiconductor elements that are integrated in a limited semiconductor substrate. In order for the individual semiconductor elements to form electric circuits, the individual elements may be electrically insulated, as required. Generally, therefore, a semiconductor element is formed in a semiconductor region that is usually called an island and that is electrically isolated by inversely biasing the pn junction. The problem here is that a voltage for inversely biasing the pn junction is applied to the semiconductor substrate, i.e., applied to the substrate of a heat-generating device that forms a contacting interface relative to the heat-conducting member. Only rarely do all of the heat-generating devices mounted on the base board have semiconductor substrates with the same function. Usually, it should be assumed that two or more heat-generating devices having dissimilar functions are mounted in the same cooling module. In such a case, it becomes necessary to maintain the inverse biasing voltages at two or more levels. However, the heat-generating devices served with inverse biasing voltages of different levels tend to be electrically communicated with one another via the heat-conducting member which is electrically conductive, elastic member, and cap so that predetermined inverse biasing is not maintained, and the cooling module malfunctions. The above-mentioned problem can be eliminated when all of the heat-generating devices mounted in the cooling module, have the same inverse bias. However, the probability remains that the cooling modules may be electrically communicated with each other via a heat sink that forms a contacting interface relative to the cap or via impurity substances or contaminating substances contained in the cooling agent, or that an electric network is formed by cooling modules that come into contact with each other when they vibrate on the printed boards that are very densely mounted in the housing. Therefore, the heat-generating devices mounted on the base board should not be electrically connected except in predetermined electrically conductive paths. For this reason, the cap or the heat-conducting member should be provided with electrical insulating properties.
A principal object of the present invention is to provide a heat-conducting cooling module having heat-conducting relay members or housings that efficiently cool the heat-generating devices that are to be cooled, and that maintain heat-conducting performance of heat-conducting paths from the heat-generating devices to a heat sink that radiates the heat.
Another object of the present invention is to provide a lightweight heat-conducting cooling module which prevents or reduces the deterioration of the connection pins and of the connection members, and to provide a lightweight heat-conducting cooling module which prevents or reduces deterioration of or damage to the connection members that electrically connect the heat-generating devices to the wiring of the base board or prevents or reduces the deterioration or damage to the heat-generating devices.
A further object of the present invention is to provide a heat-conducting cooling module equipped with heat-conducting relay members or housings which are resistant to wear and which have electrically insulating properties to prevent or reduce wear and conductivity among the electrically conductive paths, and to prevent electrical communication among one or more heat-generating devices to which a voltage is applied.
The heat-conducting cooling module of the present invention comprises at least one heat-generating device that is to be cooled, a housing contained in a heat-conducting path so as to be opposed to the heat-generating device, a heat-conducting relay member arranged so as to exchange the heat with respect to the surface wall of the housing, and resilient means for pressing the heat-conducting relay member onto heat-generating device to form a heat-conducting interface therebetween, wherein the housing or the heat-conducting relay member is made of a sintered product which consists of silicon carbide as a chief component.
According to the heat-conducting cooling module of the present invention, the housing or the heat-conducting relay member is made of a sintered product which consists of silicon carbide as a chief component, and which contains at least one of beryllium, beryllium oxide or boron nitride.
In the heat-conducting cooling module of the present invention, the housing or the heat-conducting relay member is made of a sintered product which includes of silicon carbide as a chief component, and which contains at least one of beryllium, beryllium oxide or boron nitride, and at least one of aluminum, silicon, iron, titanium or nickel, or oxides thereof or carbides thereof.
Moreover, the heat-conducting cooling module of the present invention comprises at least one heat-generating device that is to be cooled, a housing which has protrusions opposed to each of the heat-generating devices and which is contained in a heat-conducting path, a heat-conducting relay member which has opening portions in which the protrusions are arranged so that the heat is exchanged between the side walls of the opening portions and the surface walls of the protrusions, and resilient means that presses the heat-conducting relay member onto said heat-generating device to form a heat-conducting interface therebetween.
According to the heat-conducting cooling module of the present invention, the housing or the heat-conducting relay member is made of a sintered product having resistivity greater than 10.sup.10 ohms-cm
According to the heat-conducting cooling module of the present invention, the thermal resistance of the heat-conducting path is adjusted in dependence upon the opposing areas between the surface wall of the projections of the housing and the side walls of the opening portions of the heat-conducting relay member.
The above-mentioned heat-conducting relay member and the housing has excellent heat conductivity which is nearly comparable to that of metals such as copper, aluminum, is as lightweight as possible, has resistance against wear, and has electrically insulating properties. It has been experimentally determined that a material having the above-mentioned properties includes a sintered silicon carbide having high density containing at least one of beryllium, beryllium oxide or boron nitride. More specifically, it has been determined that a sintered silicon carbide containing more than two parts by weight of beryllium (reckoned as beryllium oxide) or boron nitride per 100 parts by weight of silicon carbide exhibits thermal conductivity of 0.7 cal/.degree. C.cm.S (at room temperature), a density of 3.2 g/cm.sup.3, a Vickers' hardness of about 4000, a bending strength (supported at three points) of 45 kgf/mm.sup.2, and an electric resistivity of 10.sup.13 ohms-cm or more (at room temperature), is suited for use as the heat-conducting relay member and the housing material.
The heat-conducting relay member and the housing made of the sintered silicon carbide exhibit increased electric resistance at the silicon carbide bonded grain boundaries owing to the presence of beryllium, beryllium oxide or boron nitride, exhibits electrical insulating properties inherent in the sintered product of silicon carbide, and is further imparted with thermal conductivity. In the sintered silicon carbide are remaining silicon, aluminum, iron, titanium and nickel, or oxides thereof, carbides thereof, as well as free carbon, that had been contained in the form of impurities in the starting materials. Among these impurities, aluminum decreases the resistivity of the sintered product of silicon carbide and should, therefore, be contained in small amounts. Aluminum also plays an important role in increasing the density of sintered silicon carbide, i.e., to decrease the porosity. The decrease in the porosity is significant in the process of sealing the package, as will be described later. In such a case, therefore, it is desired to add beryllium, beryllium oxide, or boron nitride in amounts to compensate for the reduction resistivity caused by aluminum.
In the heat-conducting cooling module of the present invention, the resistivity required for the heat-conducting relay member and the housing should be greater than 10.sup.10 ohms-cm, and thermal conductivity should be at least equal to, or greater than, that of aluminum (0.53 cal/cm..degree. C. S). To achieve this, two parts by weight or more of beryllium reckoned as beryllium oxide or boron nitride should be added to 100 parts by weight of the silicon carbide that is a chief component.
Further, the housing must sufficiently define a space for containing the heat-generating devices together with the base board or other members, and contain therein a gas such as helium for assisting in transferring the heat at the interface between the heat-conducting relay member and the housing or the heat-generating devices. This is because, helium has small atomic radii and escapes through very small gaps or pores. The hermetic seal is a serious problem, particularly when ceramic members are to be used as housing members, unlike the case of using metals. Ideally, the hermetic seal should be smaller than 10.sup.-7 atm ml/s reckoned as the leakage of helium. To maintain such a hermetic seal, the sintered silicon carbide should have a relative density greater than 97%. To obtain such a sintered product of silicon carbide, typically, a silicon carbide powder having particle diameter smaller than 2 um should be homogeneously kneaded with additives having similar particle sizes, insulating properties and heat conductivity. The mixture should then be temporarily molded at a pressure of about 98 MPa, and should be hot-pressed at a vacuum of 10.sup.-6 MPa at a temperature of 2050.degree. C., at pressure of 30 MPa, and for one hour. The sintered silicon carbide contains silicon, aluminum, iron, titanium, and nickel, or oxides thereof, carbides thereof, as well as free carbon that had been contained as impurities in the starting materials, in addition to the additives used for imparting insulating properties or thermal conductivity. These impurities work effectively to intimately bond silicon carbide crystalline particles. In order to hermetically seal the sintered silicon carbide, it is desirable to add the above-mentioned silicon, aluminum, iron, titanium, nickel, or oxides thereof, or carbides thereof.
Moreover, to efficiently exchange heat between the heat-generating devices contained in the heat-conducting path and the heat-conducting relay member and between the heat-conducting relay member and the housing, the opposing areas between them should be as wide as possible, and the gaps should be as small as possible. In this sense, the heat-conducting relay member forming the interface between the heat-generating devices and the housing should be as large as possible. However, an increase in the size of the heat-conducting relay member increases the weight thereof, causing the heat-generating devices to be deformed or damaged, and further causing the members connecting such devices to the base board to deteriorate or be damaged. That is, when the opposing areas among the heat-conducting relay member, the heat-generating devices and the housing are to be maintained the same, the weight of the heat-conducting relay member can be reduced, and the heat-generating devices and the connection members can be prevented from deteriorating or being damaged when the heat-conducting relay member is arranged between the heat-generating devices and the housing having protrusions at portions opposed to the heat-generating devices, the heat-conducting relay member having opening portions at portions opposed to the protrusions, rather than when nearly a pole-shaped heat-conducting relay members are arranged between the heat-generating devices and the housing having openings at portions opposed to the heat-generating devices.